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That functionality doesn't currently exist; I've posted a feature request regarding this here: http://pkp.sfu.ca/bugzilla/show_bug.cgi?id=6615. The Crossref documentation isn't the clearest, but I think I've got most of the major points in that report. If you have any comments on how this might work or look, please feel free to comment either here, or on the report. I'm not sure when we'll be able to get to this -- it's a non-trivial development task, and we're currently busy with other things. But it would be a cool feature to include.